Jabber @ RWTH - Present
The current system behind jabber.rwth-aachen.de is based on Debian/GNU Linux "Stretch" as the operating system, ejabberd from the official Debian Stretch Backports as the XMPP server and the Django based homepage from jabber.at, modified and extended for the RWTH Aachen University.
It has replaced the previous system consisting of Openfire, WordPress and handwritten PHP code. All relevant data was migrated during the process. The developed migrations scripts are published at github.com.
Both Django and Ansible are written in Python, ejabberd is written in Erlang. The migrations scripts are also written in Python.
Fancy statistics for the current system can be found here (only accessible from within the RWTH network).